This invention relates to methods of preparing environmentally stable bonded aluminum structure and more particularly relates to methods of preparing bonded aluminum structures in which the aluminum surface is rendered especially well adapted to receive the adhesive resin and is resistant to subsequent delamination and failure of the adhesive bond at the adhesive resin-aluminum interface.
The structural bonding of metal to metal and composite type assembly widely used in the aircraft industry and elsewhere frequently require a resultant structure which is reasonably resistant to the extremes of atmospheric conditions found in use. For example, in aircraft construction the wing structure utilized in manufacture of large passenger, cargo, and military aircraft, utilizes adhesively bonded structures which are subjected to extremes of temperature varying from substantially below zero Farenheit in Arctic areas to temperatures in excess of 150.degree. F. in tropical areas when the aircraft must be exposed to the tropical sun. Aircraft are also exposed to marine environments and other highly corrosive atmospheres. To avoid failures of the aircraft structures as well as to meet the stringent requirements of the military aircraft standards and the standards established by the airfraft industry for commercial passenger and cargo aircraft, bonded metal to metal and composite type assemblies must be able to withstand the environmental conditions to be encountered. Of particular importance is resistance to corrosion and delamination of composite structures occasioned by humid warm environments which attack prior art materials. Heretofore, the adhesively bonded metal-to-metal and composite type assemblies have performed less than satisfactorily due to adhesive failure at the interface between the polymeric adhesive and the aluminum surface, frequently necessitating field repairs and occasionally removal of the aircraft from service so that extensive repairs may be undertaken.
It is well known that aluminum or aluminum alloy surfaces exhibit unpredictable and unreliable adherence to bonding media particularly in moist and salt laden atmospheres. It has been proposed to increase adherence of surface coating such as electroplated metal on aluminum base by means of an anodic treatment in an acid bath and then dissolving a portion of the oxide film in an acid or alkaline bath prior to electroplating. See U.S. Pat. No. 1,971,761. It has also been proposed to electroplate directly over an oxide film produced by anodizing aluminum or aluminum alloys in chromic acid or phosphoric acid solution without intermediate treatment of the oxide film such as is taught in U.S. Pat. Nos. 1,947,981, 2,036,962 and 2,095,519. In each of the above-noted patents the aluminum surface is being prepared for electroplating.
Similarly, it has been proposed in U.S. Pat. No. 3,672,972 to form anodic coatings having improved adhesive properties on aluminum surfaces by depositing coatings on the aluminum substrate by subjecting the aluminum article to electrolytic treatment in an aqueous solution of various acids such as phosphoric acid, oxalic acid, sulphuric acid, malonic acid and the like at elevated temperatures for a very short treatment period. Similarly, it is known to treat oxides already formed on an aluminum surface by other means with a phosphate bath electrolysis to render the oxide surface hydration resistant. The elevated temperature phosphoric acid anodization process results in the deposition of an oxide surface characterized as "pseudoboehmite," a highly active form of aluminum oxide deposited in a very thin, nonporous and uniform layer on an aluminum surface. The characteristics of this form of aluminum oxide apparently permit failure within the oxide structure when high stressed under humid conditions. In addition, lag time after cessation of anodizing current encountered in commercial processing of aluminum surfaces at the elevated temperatures (95.degree.-122.degree. F.) of this patent cause dissolution of the aluminum surface by the phosphoric acid electrolyte. Poor bonding results wherever the aluminum surface is excessively dissolved.